The present application describes systems and techniques relating to load balancing in a data delivery system, for example, balancing server load in a server cluster.
A network is a collection of nodes coupled together with wired or wireless communication links, such as coax cable, fiber optics or radio frequency bands. Each node is capable of communicating with other nodes over the communication links using networking protocols. A node may be any machine capable of communicating using the network protocol. An inter-network is a collection of computer networks coupled together by routers (also known as gateways) and an inter-networking protocol.
A server cluster is a group of independent data servers, or nodes, coupled with a network and managed as a unified data delivery system. The servers in the cluster cooperate in providing data to requesting client devices. A load balancer or redirector may be used to distribute client requests across the servers within the cluster. Distributing load across different servers allows the server cluster to handle large numbers of concurrent requests and large volumes of requested data, while keeping response time to a minimum.
A load balancer commonly performs two interrelated functions: (1) server selection and (2) request translation. Request translation refers to the conversion of a client request directed to the server cluster generally, into a specific request directed to an individual server within the cluster. Server selection involves choosing an individual server to process a particular client request.
A common example of a server cluster is a web server cluster. A web server provides users with access to data, which is typically in the form of HTML (Hypertext Markup Language) documents and software organized into a web site. Each web server in a conventional web server cluster typically stores identical web site content and typically runs mirroring software to maintain the duplicate content across all the servers in the cluster.
With regard to request translation, traditional approaches to load balancing in a web server cluster include a Domain Name Server (DNS) approach and a reverse proxy approach. In the DNS approach, a local DNS distributes server load by dynamically resolving a domain name for the cluster into different Internet Protocol (IP) addresses for the web servers in the cluster. In a reverse proxy approach, a reverse proxy server translates relative Universal Resource Locators (URLs) in client requests into absolute URLs addressed to one of the web servers in the cluster.
With regard to server selection, a traditional approach, referred to as “round robin,” assigns each server in the cluster a position in a logical circle. As each new client request comes in, the load balancer directs the request to the server associated with the next position in the logical circle.
Details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features and advantages may be apparent from the description and drawings, and from the claims.